CSS Classes, as we know are the main drivers in the UI. Since we have only one id per element, our arena or tweak on that area is almost limited.
But classes are possible to be deployed multiple times per element and we can apply independent actions per classes..
Now, when you have two classes assigned and you want to single out that element having two classes in jQuery you would do

$('.class1.class2')

Without any space in between.
Common usage would be with javascript where you would take and put classes based on actions.